Process for the production of colored protective coatings on articles of aluminum or aluminum alloys

ABSTRACT

A process is applied for the production of colored, protective coatings on articles of aluminum or aluminum alloys by passing an alternating current between a previously anodically oxidized aluminum article and a counter-electrode immersed in an acid aqueous bath containing salts of metals capable of coloring the aluminum oxide layer, the alternating voltage supplied being modulated as to its amplitude and/or frequency so as to make it asymmetrical thereby to control the color tone on the aluminum article. The asymmetrical voltage may be produced, for example, by supplying two or more alternating voltages of different amplitude, frequency and/or phase, or by using a direct voltage superimposed on the alternating voltage.

0 United States Patent 1 1 3,669,856 Gedde June 13, 1972 [54] PROCESSFOR THE PRODUCTION OF 2,901,412 8/1959 Mostovych et a1 ..204/58 COLOREDPROTECTIVE COATINGS 2,443,599 6/1948 Chester ....204/58 0N 2,231,3732/1941 Schenk ALUMINUM ALLOYS FOREIGN PATENTS OR APPLICATIONS [72]Inventor: Ove Christopher Gedde, Gaupefaret 24, 741,753 5/1943 Germany..204/58 1713 Gralum, Norway 69,930 1/1946 Norway ....204/58 Filed: June1969 662,063 4/1963 Canada ..204/58 211 App] 35 Primary ExaminerJohn H.Mack Assistant ExaminerR. L. Andrews Attorney-Waters, Roditi, Schwartz &Nissan [30] Foreign Application Priority Data June 21, 1968 Norway..2459/68 [57] ABSTRACT A process is applied for the production ofcolored, protective [52] US. Cl ..204/58, 204/128 coatings on articlesof aluminum or aluminum alloys by [51] ..C23b 9/02, BOlk 1/00 passing analternating current between a previously anodically 5 8] Field of Search..204/58, 128 oxidi ed alumin m a ticle d a cou e -elect de immersed inan acid aqueous bath containing salts of metals capable of 56]References Ci coloring the aluminum oxide layer, the alternating voltagesupplied being modulated as to its amplitude and/or frequency UNITEDSTATES PATENTS so as to make it asymmetrical thereby to control thecolor tone on the aluminum article. The asymmetrical voltage may be3,418,222 12/1968 Schaedel ..204/58 produced, for example by Supplyingtwo or more alternating 1 6O 5/1968 A Sada "204/58 voltages of differentamplitude, frequency and/or phase, or by L978 9/1960 D 'ckson et 204/58using a direct voltage superimposed on the alternating voltage.2,951,025 8/1960 Mostovych et al. ..204/58 2,935,454 5/1960 Tokumoto..204/45 7 Claims, 4 Drawing Figures PROCESS FOR THE PRODUCTION OFCOLORED PROTECTIVE COATINGS ON ARTICLES OF ALUMINUM OR ALUMINUM ALLOYSBACKGROUND OF THE INVENTION 1. Field of the Invention The presentinvention relates to a process for the production of colored protectivecoatings on articles of aluminum or aluminum alloys which havepreviously been provided with a protective oxide layer.

2. Description of the Prior Art With the increasing use of articles ofaluminum or aluminum alloys both for indoor and outdoor use, a number ofprocesses have been developed for surface treatment of such articles inorder to make them decorative and/or resistant to abrasion andatmospheric influence.

Thus, there was early developed a process for chemical coloring ofaluminum alloys with aniline dyes. The thus colored articles had poorresistance towards atmospheric influence. The coloring was carried outon aluminum articles which had previously been anodically oxidized.

There has further been developed a process whereby aluminum articles aresubjected to anodic oxidation and immersed in chemicals which penetrateinto the pores of the oxide layer, whereupon the thus treated aluminumarticle is placed in aqueous solutions of salts which also penetrateinto the pores and there combine with the first used chemical. Thepractical realization of this process has proven difficult, and theprocess is essentially of theoretical interest only.

Processes have also been developed for simultaneous anodizing andcoloring of aluminum articles. With these processes only a limitedselection of colors is obtainable. The processes are expensive anddifficult to carry out, and very rigid requirements are made for theworking and heat-treatment of the aluminum articles, as the metallicstructure therein is of the utmost importance for the result obtained.The processes also demand the use of large currents and high voltagesand a long time, and they are relatively expensive.

From German Pat. application No. 562,615 it is known to impart a coloredcoating on aluminum or aluminum alloys by means of an acid bathcontaining chromic acid, wherein an article of aluminum or aluminumalloy having a previously applied oxide layer is used as acounter-electrode. The counter electrode and the article to be coatedare immersed in the bath containing chromic acid, and an alternatingcurrent is passed between the counter-electrode and the article to whichthe protective coating is to be applied. This method is uneconomical andoffers few possibilities for selection of colors.

According to German Pat. specification No. 655,700 aluminum articleswhich have already been provided with an oxide layer may be providedwith a further, opaque, white, protective layer by electrolytictreatment of the article with alternating current or direct current inan aqueous bath containing dissolved titanium salts. The deposited,opaque film may then be colored with organic dyes in aqueous or organicsolvents. The process is expensive and cumbersome.

British Pat. specification No. 1,022,927 relates to a process for theproduction of colored, protective coatings on articles of aluminum oraluminum alloys by passing an alternating current through an aqueousbath containing a salt yielding a colored metal oxide or hydroxidebetween a previously oxidized article of aluminum or aluminum alloy anda counterelectrode of carbon, carborundum or aluminum. This process islimited to the use of particular counter-electrodes in order to obtainthe colored coatings, and the process has little flexibility as regardsthe obtainable color tones.

BRIEF SUMMARY OF THE INVENTION It has now suprisingly been found that bypassing an alternating current between a previously anodically oxidizedarticle of aluminum or aluminum alloy and a counter-electrode immersedin an acid aqueous bath containing metal salts having coloring cationsthe color tones of the coatings can be controlled in a simple manner bymodulating the shape of the curve of the applied alternating voltage insuch a manner that during the coloring process the alternating voltagewill provide a suitable ratio between the two current directions for anadvantageous transport of material and course of reaction with regard tothe previously anodically oxidized aluminum article.

The present invention thus provides a process for the production ofcolored protective coatings on articles of aluminum or aluminum alloysby passing an alternating current between a previously anodicallyoxidized aluminum article and a counter-electrode immersed in an acidaqueous bath containing salts of metals capable of coloring the aluminalayer, and the process is characterized in that the alternating voltagesupplied is modulated as regards its amplitude and/or frequency so as tomake it asymmetrical thereby to control the color tone on the aluminumarticle.

The modulation of the alternating voltage can be carried out in severalways, e.g., by simultaneously supplying two or more differentalternating voltages or a superimposed direct voltage or by generatingan alternating voltage having the desired frequency and curve shape.

As the material for the counter-electrode, stainless steel or titaniumare preferably used because they are practically insoluble in the acidbath used for the coloring and because they lead to an advantageouslylow energy consumption.

It is assumed that the incorporation of metal salts in the oxide layerof the aluminum article is due to deposition and dissolution of metal atthe surface of the aluminum article caused by the alternating current asthe current changes its direction, however, in such a manner that someof the metal forms chemical compounds with other substances, e.g.,oxygen, sulphur, hydroxyl groups or other groups, and remains in thepores of the aluminum oxide layer.

The strength of the alternating voltage and the modulation of theamplitude and/or frequency thereof according to the present process isfrom 5 to 50 volts dependent upon the composition of the electrolyte andthe properties of the oxide layer previously formed. Preferably there isused a current density of 0.1 to 0.5 A/dm, dependent upon theelectrolyte employed, and a low treatment period of 1 to 10 minutes.

BRIEF DESCRIPTION OF THE DRAWING FIG. la schematically shows a circuitdiagram which can be used to modulate the applied alternating voltageaccording to the present process;

FIG. lb shows the curve of a such modulated alternating voltage;

FlG.2a shows an alternative embodiment of a circuit diagram which can beused for supplying a modulated alternating voltage in the presentprocess; and

FIG. 2b shows one form of the curve of an alternating voltage modulatedby using the circuit of FIG. 2a.

DETAILED DESCRIPTION In FIG. 1 S and S are two oppositely directedrectifiers coupled in parallel relationship with regard to one another,each rectifier being connected in series with a rheostatR and RSubstantially the same circuit diagram as in FIG. 1 is shown in FIG. 2,however, here two controlled rectifiers T and T are used. These arearranged such that in a controllable manner they can be respectivelyused to disrupt the current in a part of each of the positive andnegative half periods of the alternating current. It is then possible toomit the rheostats shown in FIG. 1, and there is obtained a control ofthe two currents in respectively positive and negative direction throughthe electrolysis bath essentially without any energy losses.

In order to be able to regulate these two currents in the positive andnegative directions and the pertaining potentials between the electrodesof the electrolysis vessel, the circuits are provided with an ammeter Ain series with each rectifier and with two voltmeters V connected inseries respectively with oppositely directed rectifiers corresponding indirection with the rectifiers S and S or T, and T The circuits shown canbe used for voltages of from to 200 volts, preferably 0 to 50 volts, forcurrent strengths of from 0 to 10,000 A, preferably 0 to 1,000 A, andfor a frequency of 5 to 500 cycles per second, preferably 50 cycles persecond.

Since in the rheostats used in the circuit according to FIG. 1 anappreciable loss of electric energy will occur, it is preferred to use acircuit as shown in H6. 2 and comprising controlled rectifiers. By theterm controlled rectifiers" as used herein is meant such rectifierswhich can be made current carrying by the use of an auxiliary potentialwhich is synchronous with the main voltage, i.e., thyristors. By the useof two such thyristors in parallel it is possible in a controllablemanner to cut off a part of the positive and negative half waves of thealternating current. Thus, it is possible to regulate the effectivevalue of the two half waves.

In the present process it has been found by the use of a number ofelectrolytes that it is possible in a simple manner to control theamount of the deposited, coloring compounds in the oxide layer and thecolor tone caused thereby. The colored coating is very resistant towardsabrasion and has a very high resistance towards atmospheric influence,and the aluminum articles produced by the present process have proven tobe particularly well suited for outdoor use.

EXAMPLE 1 An aluminum article with a previously oxidized surface wasimmersed in an aqueous bath containing 2.0 percent sulphuric acid, 7percent aluminum sulphate and 1.5 percent copper sulphate. An electrodeof stainless steel was used as a counterelectrode.

Alternating current was passed through the electrolyte at a voltage of20 volts between the aluminum article to be colored and thecounter-electrode. The current density was 0.4 Aldm The appliedalternating voltage was made asymmetrical from two different A.C.sources applied simultaneously and having different phases in such amanner that the total negative half wave of the alternating voltage withregard to the aluminum was greater than the positive one. A deep redcolor was obtained in 10 minutes.

EXAMPLE 2 A previously oxidized aluminum article, the samecounterelectrode and the same electrolyte as in Example 1 were used.

The alternating current was now supplied in such a manner that thepositive half wave of the alternating voltage was greater than thenegative one.

The coloring was carried out in the same time and with the same voltageand current density as in Example 1. There was obtained a red colorwhich was less intense than the color obtained in Example 1.

EXAMPLE 3 An aluminum article which had previously been anodicallyoxidized in aqueous sulphuric acid was immersed in an electrolytecontaining 1.5 percent boric acid and 1 percent cadmium sulphate. A leadelectrode was used as a counter-electrode.

During the first 10 seconds of the coloring, a symmetrical alternatingvoltage was supplied to the aluminum article to be coated and to thecounter-electrode. The alternating voltage was then modulated so as tomake its negative half wave greater with regard to the lead electrodethan its positive half wave. The alternating voltage was madeasymmetrical by superimposing thereon a rectified alternating voltage.

The current strength remained substantially constant, and a deep, blackcolor was obtained on the aluminum.

The current density used was adjusted with regard to the alumina layerand was 0.4 A/dm. if too high current densities are used the previouslyformed oxide layer on the aluminum will be damaged. The treatment periodwas 10 minutes.

EXAMPLE 4 An aluminum article which had previously been anodicallyoxidized in aqueous sulphuric acid was treated in the same electrolyteas in Example 3, and the same counter-electrode, alternating voltage andcurrent density were used.

During the first 2 minutes a symmetrical alternating voltage was used,and the alternating voltage was then made asymmetrical and the coloringcontinued for 8 minutes. Thus, the total treatment took 10 minutes. Abrown color was obtained on the aluminum article.

EXAMPLE 5 An aluminum article which had previously been anodicallyoxidized was used in an electrolyte together with a counterelectrode asdescribed in Example 3. The same alternating voltage and current densitywere also used. The alternating voltage supplied was modulated from thebeginning so as to be asymmetrical, and after a treatment period of 10minutes there was obtained a beige color on the aluminum article.

The aluminum articles colored by using the present process have provento possess a coating which is very resistant to abrasion and atmosphericinfluence. By means of the modulation of the amplitude and/or frequencyof the applied alternating voltage according to the present process, thecolor tone of the aluminum article can also be varied within a verybroad range of color tones, and the necessary time for obtaining acolored coating which is very resistant towards abrasion and atmosphericinfluence can be kept very short. There is thereby obtained aconsiderable saving both as regards labor and ener gy consumption.

It is particularly preferred initially and for a brief period to supplya symmetrical alternating voltage and then an asymmetrical alternatingvoltage.

In addition, the coloring takes place faster and more efficiently if thealternating voltage is regulated relatively slowly, of the order of afew seconds, from zero to the voltage which is decided for the coloring.This relates both to the starting up of the coloring and to a latersupply of another alternating voltage than the one initially used.

What is claimed is:

l. in a process for the production of colored, protective coatings onarticles of aluminum or aluminum alloys, said process comprising passingan alternating current between an electrode system comprising apreviously anodically oxidized aluminum article and a counter-electrodeimmersed in an acid aqueous bath containing salts of metals capable ofcoloring the aluminum oxide layer, the improvement wherein analternating voltage with controlled asymmetry is applied to theelectrodes whereby to control the color tone of the aluminum article,the alternating voltage having been made asymmetrical by modulating thealternating voltage externally of said electrode system.

2. A process claimed in claim 1, wherein initially a symmetricalalternating voltage is passed followed by the asymmetrical alternatingvoltage.

3. A process as claimed in claim 1, wherein the asymmetrical voltage isproduced by supplying at least two alternating voltages of differentamplitude, frequency and/or phase.

4. A process as claimed in claim 1, wherein the asymmetrical alternatingvoltage is produced by superimposing a direct voltage on the alternatingvoltage.

5. A process as claimed in claim 1, wherein the alternating voltage isregulated relatively slowly from zero to the voltage used for thecoloring.

6. A process as claimed in claim 1, wherein the alternating current ispassed through two oppositely directed rectifiers connected in paralleleach connected in series to a rheostat, and regilating the current bymeans of the two rheostats.

7. A process as claimed in claim 1, wherein the alternating current ispassed through two oppositely directed controlled rectifiers connectedin parallel, and controlling the rectifiers so as to disrupt the currentduring a pan of each of the positive and negative half waves of thealternating current.

2. A process claimed in claim 1, wherein initially a symmetricalalternating voltage is passed followed by the asymmetrical alternatingvoltage.
 3. A process as claimed in claim 1, wherein the asymmetricalvoltage is produced by supplying at least two alternating voltages ofdifferent amplitude, frequency and/or phase.
 4. A process as claimed inclaim 1, wherein the asymmetrical alternating voltage is produced bysuperimposing a direct voltage on the alternating voltage.
 5. A processas claimed in claim 1, wherein the alternating voltage is regulatedrelatively slowly from zero to the voltage used for the coloring.
 6. Aprocess as claimed in claim 1, wherein the alternating current is passedthrough two oppositely directed rectifiers connected in parallel eachconnected in series to a rheostat, and regulating the current by meansof the two rheostats.
 7. A process as claimed in claim 1, wherein thealternating current is passed through two oppositely directed controlledrectifiers connected in parallel, and controlling the rectifiers so asto disrupt the current during a part of each of the positive andnegative half waves of the alternating current.